Conventionally, commodity chemicals have been made largely from fossil or petroleum-based hydrocarbons. In recent years as interest in renewable resources has grown, advances in more efficient technologies have enabled renewable or “green” feedstocks to become viable economic substitutes. One class of renewable biologically-derived hydrocarbon resources has been derived from sugars. However, because of the inherent complex functionalities of sugar molecules which can readily degrade, particularly at higher temperatures, these materials have traditionally received limited attention for industrial uses.
Recent research efforts have concentrated on the production of sugar-based feedstocks with less functionality. Furans are a variety of versatile molecular platforms that can be derived readily from sugars. These molecules have structural features particularly useful in preparing polymers, pharmaceuticals, or solvents, among other industrial constituents.
A compound that has attracted attention is 5-(hydroxylmethyl)-furfural (HMF), the principal dehydration product of fructose, a cheap and plentiful monosaccharide (Scheme A).
The dual alcohol and aldehyde moieties of HMF enable it to be used as precursor for many potential industrial materials such as polymers, solvents, surfactants, and pharmaceuticals.
A drawback to employing HMF directly has been its predisposition to readily polymerize and oxidatively degrade in the presence of air, hence necessitating the presence of an antioxidant to improve longevity. HMF can be reduced to more stable molecules, such as furan-2,5-dimethanol (abbreviated as FDM) and bis-2,5-(hydroxymethyl)-tetrahydrofuran (abbreviated as bHMTHF), illustrated in Scheme B.

FDM is produced from partial hydrogenation (aldehyde reduction) of HMF as depicted in Scheme C, while exhaustive hydrogenation engenders the saturated analog bHMTHF, typically produced in a 9:1 cis to trans diastereometic proportion as in Scheme D. (See e.g., U.S. Pat. Nos. 7,317,116, or 7,393,963 B2.)


The bifunctional nature of these compounds enables them to be readily deployed as starting materials in a various chemical syntheses, and as practicable replacements for petroleum-based aromatic hydrocarbons. These materials can be valuable precursors, for example, to polyesters, polyurethane foams, plasticizers, resins, surfactants, dispersants, lubricants, agricultural chemicals, or as a solvents, binders, or humectants. The hydroxymethyl appendages at the 2 and 5 loci of the tetrahydrofuran ring provide two chiral centers, which permit bHMTHFs to be possible scaffolds for pharmaceuticals or chiral auxiliaries in the emerging realm of asymmetric organic synthesis. The applications of FDM and bHMTHFs demands both cost effective and streamlined methodologies that afford large scale manufacture of these compounds from HMF.
Exploration into preparation of molecular derivatives using FDM and/or bHMTHFs has been constrained by the prohibitive cost (e.g., ˜$200 per gram commercially) of these raw materials. To effectively compete with chemical precursors derived from fossil-based hydrocarbon sources, the preparation of HMF derivatives from common agricultural sources requires better ways of converting and producing desired derivative and intermediate compounds. Until recently, large-scale commercialization of furanics has been comparatively cost inefficient.
One approach to enhance the use FDM and bHMTHF as starting platforms for precursors or feedstocks is to convert them to esters. The established commercial synthesis of esters typically entails direct alcohol acylation with carboxylic acids catalyzed by a Brønsted acid. This protocol is commonly specified as the Fischer-Speier esterification. Typically, strong inorganic acids such as H2SO4 and HCl are employed as the catalyst. These strong acids are readily obtained, inexpensive materials but are difficult to regenerate, which increases the waste stream. Additionally, these acids can react in an undesired manner by the addition of their anionic moiety forming byproducts such as sulfate esters.
Although some robust processes have evolved recently where higher purities are attained. (See generally, X. Tong et al., “Biomass into Chemicals: Conversion of Sugars to Furan Derivatives by Catalytic Processes,” APPLIED CATALYSIS A: GENERAL 385 (2010) 1-13.), the problem of developing more efficient purification still persists.
Efforts to overcome these issues using solid resin catalysts have been tried but have been unsuccessful for large volumes. Unfortunately, traditionally employed solid acids are not hydrolytically stable and even trace amounts of water can negatively impact the catalytic activity. Homogeneous metal catalysts have also shown limited activity owing to their susceptibility to hydrolyze, which reduces the catalytic activity.
In view of these drawbacks with convention processes a need still exists for a process in which one can attain higher ester yields at economical catalyst loadings for the preparation of HMF-derived compounds for use as precursors.